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1. Field of Invention
The invention relates to the drivetrains for full-suspension bicycles.
2. Description of Prior Art
The use of different-sized sprockets between the pedals and the drive wheel of a bicycle to increase the speed of the bicycle has been well-known for many years in the prior art. In the use of rear suspension for the bicycle, many attempts have been made in the prior art to isolate the transmission from the motion of the suspension arm, thus eliminating unwanted pedal-induced motion of the suspension.
Many of the attempts made in the prior art depend on a particular relative angular position of the rear-suspension arm and the rest of the bicycle frame. Thus when this position varies in the course of normal use, the effectiveness of the mechanism in achieving the desired objective is compromised.
U.S. Pat. Nos. 5,205,572 and 5,226,674 to Buell, et al. (1993), and U.S. Pat. No. 5,217,241 to Girvin (1993) are examples of the most basic configuration for a bicycle rear suspension, that of a xe2x80x9cswingarmxe2x80x9d attached to the main frame by a pivot and providing a circular trajectory of the rear axle relative to the main frame. Usually an attempt is made to minimize the influence of the drivetrain upon the suspension by placing the pivot along or close to the line (when viewed from the side) of the part of the chain under tension. Again such a configuration is susceptible to the changing of the chain position due to the shifting of the chain between sprockets. Such a problem is present also with the double spring assembly design described in U.S. Pat. No. 6,131,934 to Sinclair (2000).
Attempts have been made to reduce this effect by creating a non-circular trajectory for the rear wheel relative to the main frame. This trajectory is usually created by means of a four-bar linkage. U.S. Pat. No. 4,789,174 (1987) and U.S. Pat. No. 5,121,937 (1992) to Lawwill show rear suspension configured in a trapezoidal arrangement, which creates a trajectory of the rear wheel relative to the main frame so as to reduce the action of the pedaling force and resultant chain tension upon the suspension. However, when the chain is shifted between the drive and rear sprockets, the varying position and angle of the part of the chain under tension, makes the influence of the tension upon the suspension unavoidable. Four-bar linkages are also described in U.S. Pat. No. 5,306,036 (1994), U.S. Pat. No. 5,409,249 (1995), and U.S. Pat. No. 5,441,292 (1995) to Busby, U.S. Pat. No. 5,678,837 (1997) to Leitner, and U.S. Pat. No. 6,102,421 to Lawwill, et al. (2000).
A different way of achieving a non-circular trajectory of the rear wheel is also described in U.S. Pat. No. 5,509,679 (1996) and U.S. Pat. No. 5,899,480 (1999) to Leitner. Here the trajectory is created by making the shock absorber itself a structural part of one of the members of the linkage. This design has the same disadvantages, described above, as does the four-bar linkage.
A different attempt to isolate the drivetrain from the suspension has been made by placing the entire drivetrain on the pivoting chainstay. This eliminates the influence of the drivetrain upon the suspension, as the relative position of the drive sprockets and rear sprockets is fixed. Such a configuration is described in U.S. Pat. No. 5,474,318 to Castellano (1995), U.S. Pat. No. 5,685,553 to Wilcox, et al. (1997), and U.S. Pat. No. 6,109,636 to Klein (2000). However, such a configuration makes the stiffness of the suspension when the rider is seated different from that when the rider is standing. Such variation in stiffness is actually the aim of some suspension designs, such as U.S. Pat. No. 5,611,557 to Farris, et al. (1997). Such a design uses this variation to reduce the energy lost due to the xe2x80x9cbobbingxe2x80x9d which occurs when the rider is standing and pedaling hard. Again, such a solution is reached at the cost of increased stiffness while standing, which causes fatigue in the rider.
Other inventions involve an eccentric crank mechanism. This causes compression of the suspension to effectively lengthen the chainstay to counterbalance the pedaling force of the rider. This is described in U.S. Pat. No. 5,553,881 (1996) and U.S. Pat. No. 5,628,524 (1997) to Klassen, et al., and U.S. Pat. No. 6,099,010 to Busby (2000). This configuration suffers from the fact that the pedaling force to be balanced varies, because of the changing terrain, because of the natural variation in applied force during the pedal stroke, and because the effective force exerted by the drivetrain upon the suspension varies with the shifting of the chain between sprockets.
Another patent, U.S. Pat. No. 5,725,227 to Mayer (1998)xe2x80x94describes a configuration whereby the seat is mounted on an intermediate frame member, between the front portion which includes the steering tube, and the rear portion which includes the drop-outs holding the rear wheel. Such a configuration does not eliminate the problem of variable chain position present with the basic swingarm design.
Some designs make the pivot of the swingarm coaxial with that of the pedal cranks, as does the present invention. This is seen in U.S. Pat. No. 6,149,175 to Fujii (2000). But without the means, as in the present invention, whereby the motion of the swingarm induces like motion of the drive sprocket, this is essentially the same configuration as that where the entire drivetrain is mounted on the swingarm. The same fixed chain length is created, but so is the same variation of stiffness.
In accordance with the present invention a crank assembly comprises a plurality of gears for operating the drivetrain of a bicycle, centered at and near the axis of the pedal cranks, and the proximal portions of the chainstays, which are free to rotate about the same axis.
The object of the present invention is to eliminate the influence of the drivetrain upon the suspension. Though the device involves more gears in the transmission of power than does the usual drivetrain, the efficiency will be at least as good as that of the internal multi-speed rear hub. An additional advantage of the device is that smaller chainrings are required, which increases the clearance between the large chainring and the ground. The increase in clearance and the isolation of the suspension from the drivetrain allow for rear suspension which both has long travel and is power-efficient. As well, the smaller chainrings decrease the difference in radius between adjacent chainrings, allowing easier operation of the front derailleur.